This application claims the priority of German Patent Application No. 198 34 571.2, filed Jul. 31, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for producing bodies made of fiber-reinforced composites from fiber-reinforced precursor bodies having a porous carbon matrix, in particular from C/C bodies. Fibers, fiber bundles or fiber agglomerates based on carbon, nitrogen, boron and/or silicon are mixed with at least one carbonizable binder, and if desired further additives or fillers, to give a pressing composition. The pressing composition is pressed to form a green body that is subsequently pyrolysed to convert the binder or binders into a porous carbon matrix. The resulting fiber-reinforced precursor body is infiltrated with molten metal or silicon.
The process of the present invention relates in particular to the production of ceramic composites and also to fiber-reinforced ceramic composites. A porous pre-ceramic body is infiltrated with a metal or silicon melt. In this infiltration, the liquid metal or silicon enters the pores. In the case of infiltration of pre-ceramic carbon bodies with silicon, the reaction to silicon carbide produces the actual ceramic. This process is described, for example, in DE 44 38 455 C1.
This type of process is used in the production of ceramic brake discs. However, the production of internally ventilated brake discs presents a problem. It is very difficult to introduce ventilation channels into solid, ceramic disc-like bodies since the material is very resistant and difficult to machine. Introduction of the channels into the pre-ceramic precursor body can lead to problems because the melt infiltration can sometimes alter the geometry of the workpiece and the dimensions.
It is therefore an object of the present invention to provide a process by means of which ceramic bodies having internal structures, in particular channels and the like, can be produced.
This object is achieved by providing a green body with at least one metal- or silicon-containing core that is used as metal or silicon source for the melt infiltration.
The process of the present invention in which the pre-ceramic bodies are melt-infiltrated using metal- or silicon-containing cores, with the cores serving as sources of metal or silicon, has the advantage that internally structured composites can be produced with high dimensional accuracy in a very simple manner.
In a variant of the present invention, the pre-ceramic body or green body is produced with internal structures into which the metal- or silicon-containing cores are inserted prior to melt infiltration. In a second embodiment, the internal structure, in particular the ventilation channels, are introduced in the form of metal- or silicon-containing cores during the pressing of the green body and remain in place until melt infiltration. Thus, it is possible to have (1) cores purely as the silicon source, or (2) cores which, for example in the manufacture of ventilated brake discs, function as pressing cores in the form of a tool insert to produce ventilation channels.
According to the present invention, preference is given to using cores of technical-grade metal or silicon. Another advantageous embodiment provides for a core comprising a mixture of silicon and boron nitride. In a third advantageous embodiment, alloys, in particular silicon-iron alloys or silicon-aluminium alloys, are used as the core.
The cores can be produced by casting or pressing. In addition, the cores can be produced under a protective gas. A protective gas is necessary when melting the silicon prior to the actual production of the core. Cores of high-purity to technical-grade silicon can be produced by casting. A preferred method of producing the cores is core shooting as is employed, for example, in the production of sand cores in foundry work. Silicon/boron nitride mixtures or alloys of silicon with other metals are castable and the melting point depression compared with the pure components, which is advantageous. They can also be produced by extrusion or pressure casting.
The channels to be filled with the cores can also be introduced by milling after the pressing of the green body.
In a further advantageous embodiment, a green body containing metal- or silicon-containing cores is pressed in a press with a floating die from a pressing composition containing one or more cores. During this procedure, the fibers in the pressing composition are oriented along the metal- or silicon-containing core in the direction of force. The resulting ceramic body has the typical properties of a ceramic, namely stiffness and a high abrasion resistance. These properties are particularly well-suited to brake discs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.